The present invention relates generally to lubricants for coating moving webs. More specifically, the present invention relates to an improved lubricant for coating webs moving at very high speeds.
In the paper manufacturing industry, paper and paper board web material is frequently coated to improve properties such as appearance and printability. The coating process involves applying a coating mixture to the paper as it moves at high speed through a coating apparatus. These coatings are typically composed of: (1) pigments, such as Kaolin clay, titanium dioxide, calcium carbonate or silicates; (2) adhesive binders, such as starches, styrene butadiene latex, or polyvinyl acetates; and (3) additives, which improve or modify specific properties and characteristics of the coating mixture.
A number of chemical phenomena and physical forces (particularly shear forces) can have a detrimental effect, e.g., destroying the integrity and uniformity of both the coating mixture and the coating as it is applied to the paper sheet. When this occurs, costly problems develop both in the application of the coating and in the finish quality of the coated paper. These problems are widely known to manufacturers of coated papers.
These problems are greatly magnified as the speed of the web moving through the coating apparatus is increased, causing increased shear forces on the coating mixture being applied. Increased speed causes problems in viscosity control, calcification, streaking, whiskering, and generally poor "runnability" (performance) of the coating. These problems ultimately result in a poor quality coated sheet and costly rejections. Uniformity of the coating mix at these high shear levels is critical.
Typically, lubricant additives, such as calcium stearate dispersions or polyethylene emulsions, have been used as an additive to the coating mixture to improve the performance and uniformity of the coating and the overall integrity of the coating mixture. The main function of a lubricant additive in a coating mixture is to increase the lubricity of the coating. However, many other effects of lubricants are known to the art of coating paper. For example, such characteristics as rheological properties, plasticity, smoothness, coating gloss, anti-dusting and improved printing qualities, can be affected by the use of coating lubricants.
In U.S. Pat. No. 4,766,015, issued to Nikoloff et al., attempts have been made to provide a lubricant with superior rheological properties. However, such prior art attempts do not adequately address the needs and related problems associated with high-speed printing on modem machines. The additive mixture of Nikoloff '015 includes a fatty acid, a surfactant and a crude lecithin component. However, since a fatty acid and crude lecithin are employed, a surfactant must also be employed for the mixture to be dispersible and lubricated enough to function properly as a paper coating mixture.
In U.S. Pat. No. 4,200,551, issued to Orthoefer, describes a chemically modified lecithin concentrate where various crude lecithins and surfactants are blended to form a water dispersible lecithin. Other types of chemicals modifications are also made to form hydroxylated lecithin, acetylated lecithin and others. The chemically modified lecithins disclosed in Orthoefer are particular well suited to the food industry due to the improved water dispersion properties.
The need for a superior lubricant is evident where the solid's density in a coating structure typically runs from 20.0-23.0 pounds per gallon while the liquid density typically runs from 8.0-9.5 pounds per gallon. The challenge lies in the ability to maintain the two densities in a coating structure together in one homogeneous coating even at the high speeds of greater than 6000 fpm. The modern, high-speed coating processes and equipment described above generates higher velocities than ever experienced before during application which, in turn, creates a centrifugal force that makes it more difficult to develop a stable clay coating. Therefore, there is a need to address the problems associated with the centrifugal force at which the coating is exposed during its application in maintaining the two densities together at high speed and velocities.
Recently, the foregoing concerns regarding paper coatings have become more and more important in light of modern, high-speed printing processes that have placed more and more demands on the performance of coated paper. In particular, over the last decade, the speed at which clay coatings are applied to paper have almost doubled and created the need for a better lubricant in coating moving webs. The demands of these new high-speed printing and coating processes have necessitated a change in the lubricants over the standard phospholipid (lecithin), calcium stearate or polyethylene used in the prior art. Since the speeds of modern printing processes often reach 6000 fpm, and possibly greater, superior lubricants are required to maintain the coating structure particularly where the coating colors are applied via the modem clay coating equipments. Such a superior lubricant is required to maintain the efficiency, economy and high quality from the manufacturers of coated paper while still maintaining the required optical properties of brightness, opacity and smoothness. However, there are no prior art lubricants which accomplish these goals and accommodate the modem printing equipment and processes. Therefore, an improved lubricant additive which accommodates these modem, high-speed printing machines and processes is desired to address those needs not found in the prior art.